This invention relates to a photosensitive device, an image forming apparatus and a method for forming images using them, and more specifically, to those devices and method using a long photosensitive sheet to be rolled up after usage.
A pressure transfer method is known as a prior-art transferring method for transferring the toner image formed on a photosensitive member to a medium, in order to obtain stable transferred images regardless of the sort of the medium, or of change of the environment. In the pressure transfer method, toner particles on the surface of the photosensitive member is transferred to the medium by the aid of adherence of the toner particles by pressing the photosensitive member to the medium.
For example, in a liquid-process type image-forming apparatus using liquid developer, which can realize full color images having a quality comparable to that of offset printing, the pressure transfer method is adopted. That is because this process can provide higher quality transferred images than those formed by the field transfer method that generates blur of image due to liquid carrier.
In practice, the liquid-process type image-forming apparatus produces transferred images by the pressure transfer method, in which developed images formed on the photosensitive member are transferred primarily to an intermediate transfer medium and then the developed images on the intermediate transfer medium, are transferred secondarily to the ultimate medium. When transferring is carried out by the pressure transfer method, a high pressure is generally applied to the photosensitive member. For example, a pressure of approximately 100 kgf (or 9.8 N) is applied to the photosensitive member for transferring width of 300 mm. A higher pressure and a higher temperature are required to obtain especially high transfer efficiency.
Moreover, higher transfer efficiency can be achieved by applying a shearing force as well as pressure and temperature to the contact part of the surface of the photosensitive member with the intermediate transfer medium for the pressure transfer method. Therefore, by setting a certain rotation speed ratio of the photosensitive member to the intermediate transfer medium, the intermediate transfer medium rotates together with the photosensitive member with some imposing load torque, thereby obtaining the transferred images.
Consequently, an excess load is imposed on the photosensitive member while the transferred images are obtained by the pressure transfer method, so that it is feared that life of the photosensitive member may be shortened. Furthermore, frequent replaces of the photosensitive members may result in an extreme rise of running cost, because generally a large and expensive photosensitive drum is used in the case of the liquid-process type image-forming apparatus for obtaining full color images.
A method of replacing only a photosensitive sheet but not entire drum using a long photosensitive sheet has been proposed. That is, when the photosensitive layer becomes deteriorated, spent part of the photosensitive sheet on the drum surface is wound up and a new part of the photosensitive sheet is fed to be used as disclosed in Japanese Patent Publication Numbers Sho-62-26468 and Sho-62-51472, the disclosures of which are incorporated herein by reference.
However in those references, the photosensitive sheet wrapping the drum surface is securely supported only with a feeding roll and a winding roll which winds the spent part of the photosensitive sheet. And friction coefficient between the back surface of the photosensitive sheet and the drum surface in these constructions is set as low as possible to slide easily on each other in order to facilitate winding of the photosensitive sheet.
Therefore, when transferring the developed images by the pressure transfer method is to be carried out with the above-mentioned liquid-process type image-forming apparatus, a certain load torque for imposing pressure and shearing force by the intermediate transfer medium is applied to an area. In this area, the friction coefficient between the back surface of the photosensitive sheet and the drum surface is so low that they are easy to slide on each other. Therefore, the photosensitive sheet may be off the drum or become stretched resulting in a drawback about image forming.
The present invention is intended to solve the problems mentioned above and to provide a photosensitive device and an image-forming apparatus to obtain high quality images. To this end, the photosensitive sheet and the drum are made easy to slide on each other to facilitate winding operation at the winding process of the photosensitive sheet when the photosensitive sheet for reducing running cost of the image-forming apparatus is put into practice. In addition, the photosensitive sheet is not off the drum or does not become stretched at the image-forming process to obtain high quality images. Therefore, rising of the cost can be suppressed and good image forming can be performed.